AT89C51 / AT89S51:

 

1. Protection:

Atmel's 8051 family microcontrollers (AT89) use three Lock Bits to protect the embedded Flash memory. These bits can be programmed but cannot be individually erased. Resetting the bits is only possible through a full chip erase.

Key difference: The AT89Cxx series is programmed via a parallel programmer, while the AT89Sxx series uses a serial ISP (In-System Programming) interface.

 

1.1 Lock Bits:

Three Lock Bits (LB1, LB2, and LB3) define the device's protection level.

Protection Modes:

Bit Description LB1 When programmed, disables reading and writing of the Flash program memory. LB2 When programmed, prevents further programming of the Flash memory and the lock bits themselves until a full chip erase. LB3 When programmed, it modifies the sampling mode of the EA (External Access) pin.

 

1.2 Encryption Array:

This is a unique feature of the AT89 series. The devices contain a 64-byte Encryption Array. If this array is programmed, any data read back during a "Verify" command via the programmer will be XORed with the contents of this array. This means that even if an attacker has a copy of the firmware, they cannot perform a verification without knowing the contents of the encryption array, which complicates copying.

 

1.3 Disabling Protection:

The only way to reset the Lock Bits is to perform a "Chip Erase" command. This command completely erases the entire Flash memory, the encryption array, and all three lock bits, returning the microcontroller to its unprotected factory state.

 

2. Unique Identifier:

AT89Cxx and AT89Sxx series microcontrollers do not have a unique Chip ID or serial number. Device identification is only possible through the signature bytes, which are the same for all chips of the same model.

 

Microchip Technology (8051 Architecture)

 

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